I have a few programmers under me, they are all doing very great and very smart obviously. Thank you very much.

But the problem is that each and every one of them is responsible for one core area, which no one else on the team have foggiest idea on what it is. This means that if anyone of them is taken out, my company as a business is dead because they aren't replaceable.

I'm thinking about bringing in new programmers to cover them, just in case they are hit by a bus, or resign or whatever. But I afraid that

1. The old programmers might actively resist the idea of knowledge transfer, fearing that a backup might reduce their value.
2. I don't have a system to facilitate technology transfer between different developers, so even if I ask them to do it, I've no assurance that they will do it properly.

My question is,

1. How to put it to the old programmers in such they would agree
2. What are systems that you use, in order to facilitate this kind of "backup"? I can understand that you can do code review, but is there a simple way to conduct this? I think we are not ready for a full blown, check-in by check-in code review.